sorry but andy, that was just a shameless plug, most people tend to know what possible causes of high load are, but this guy is asking how he looks around to find out what the cause was, nothing to do with the response you posted.

first place to start is /var/log/messages

look there for kernel panics/hardware warnings etc

also check for similar items in: dmesg
(type that at the shell)

check your exim logs for any mass of e-mail traffic, and check the output of ps aux just to make sure you have no rogue processes running

sometimes (although very rarely) the cpu/memory usage thing in WHM will give you a pointer (bare in mind though that it probably isnt accurate - just gives you some ideas of where to look)

If you have one, it will be worth looking at the process list when this happens, via a serial console, KVMoIP or DRAC